Fluidsynth settings

This is an attempt at adding a dialog for FluidSynth-specific settings, and was prompted by a recent forum post [1] asking about chorus/reverb settings for FluidSynth. I don't know enough about this to know if this dialog works as intended, but something like this. Perhaps.

(If nothing else, it helped turn up a couple of glitches in some GUI widgets, so something good has already come from it. :-)

[1] http://forums.scummvm.org/viewtopic.php?t=11632